Water ice formation on interstellar carbon dust: wet HAC (WHAC)

A quantitative threshold for the radiative conversion of hydrogenated amorphous carbon (HAC) to amorphous carbon (aC) has been identified from laboratory data. This threshold, which can be expressed as a critical ultraviolet exposure $$\left \langle E \right \rangle_\textrm c\simeq10 \enspace\textrm J$$ cm^-2, is reached after exposure of dust to the standard interstellar radiation field for ∼10⁶ yr when $$A_V^0=2.3$$ mag. The conversion from HAC to aC involves a change in surface structure, which may inhibit the bonding of gaseous molecules when $$A_\textrm v\enspace \lt A_V^0\cdot$$ This suggests that the threshold for ice formation on dust grains may be related to the HAC-aC conversion. We also conclude that HAC should contain an appreciable quantity of structural H₂O, which is distinct from condensed H₂O ice. Infrared absorption due to this structural H₂O should be detectable near 2.9 and 5.9 μm.